Methods: In male Sprague Dawley rats under general anaesthesia the left mental nerve was exposed and either constricted with two 6/0 chromic gut sutures (CCI, n=6) or partially ligated using 6/0 silk (PL, n=6). Behavioural testing was undertaken 6, 3, and 1 day pre-operatively and 1, 4, 6, 8, 11 and 14 days post-operatively. The lower lip and mentum on both sides of the midline were tested using cold, hot and mechanical stimuli applied in a random order. A cotton bud soaked in ethyl chloride was used to apply a cold stimulus and the total duration of bilateral grooming recorded. A radiant heat source was used to measure the latency of head withdrawal. Mechanical stimuli were applied using von Frey hairs (0.16g – 40g) and the lowest force which caused a flinch response 50% of the time was calculated. The uninjured contralateral side served as a control.
Results: In the CCI animals bilateral grooming increased after a cold stimulus to the ipsilateral side (p=0.03, ANOVA), suggesting cold allodynia. The latency of withdrawal was also increased following an ipsilateral hot stimulus (p=0.05), suggesting hypoaesthesia. There were no significant differences in response to thermal stimuli on each side in the PL group. Responses to mechanical stimuli appeared variable and inconsistent in both injury groups.
Conclusions: Preliminary data suggest that a constriction injury to the mental nerve in the rat results in cold allodynia and heat hypoaesthesia in the lower lip and mentum. There was no clear evidence for the development of mechanical allodynia.
Supported by the Medical Research Council